Access to Astronomical Catalogues

II/246 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
The 2MASS All-Sky Catalog of Point Sources
Cutri R.M., Skrutskie M.F., Van Dyk S., Beichman C.A., Carpenter J.M.,
Chester T., Cambresy L., Evans T., Fowler J., Gizis J., Howard E.,
Huchra J., Jarrett T., Kopan E.L., Kirkpatrick J.D., Light R.M,
Marsh K.A., McCallon H., Schneider S., Stiening R., Sykes M.,
Weinberg M., Wheaton W.A., Wheelock S., Zacarias N.
<University of Massachusetts and Infrared Processing and Analysis Center
(IPAC/California Institute of Technology) (2003)>
=2003yCat.2246....0CADC_Keywords: Infrared sources ; Photometry, infrared ; Surveys
Description:
The Two Micron All Sky Survey (2MASS) project is designed to close the
gap between our current technical capability and our knowledge of the
near-infrared sky. In addition to providing a context for the
interpretation of results obtained at infrared and other wavelengths,
2MASS will provide direct answers to immediate questions on the
large-scale structure of the Milky Way and the Local Universe.
To achieve these goals, 2MASS is uniformly scanning the entire sky in
three near-infrared bands to detect and characterize point sources
brighter than about 1 mJy in each band, with signal-to-noise ratio
(SNR) greater than 10, using a pixel size of 2.0". This will achieve
an 80,000-fold improvement in sensitivity relative to earlier surveys.
2MASS uses two new, highly-automated 1.3-m telescopes, one at Mt.
Hopkins, AZ, and one at CTIO, Chile. Each telescope is equipped with a
three-channel camera, each channel consisting of a 256x256 array of
HgCdTe detectors, capable of observing the sky simultaneously at J
(1.25 µm), H (1.65 µm), and Ks (2.17 µm), to a 3σ
limiting sensitivity of 17.1, 16.4 and 15.3mag in the three bands.
The 2MASS arrays image the sky while the telescopes scan smoothly in
declination at a rate of ∼1' per second. The 2MASS data "tiles" are 6
deg. long in the declination direction and one camera frame (8.5')
wide. The camera field-of-view shifts by ∼1/6 of a frame in
declination from frame-to-frame. The camera images each point on the
sky six times for a total integration time of 7.8 s, with sub-pixel
"dithering", which improves the ultimate spatial resolution of the
final Atlas Images.
The University of Massachusetts (UMass) is responsible for the overall
management of the project, and for developing the infrared cameras and
on-site computing systems at both facilities. The Infrared Processing
and Analysis Center (IPAC) is responsible for all data processing
through the Production Pipeline, and construction and distribution of
the data products. The 2MASS project involves the participation of
members of the Science Team from several different institutions. The
2MASS project is funding by the National Aeronautics and Space
Administration (NASA) and the National Science Foundation (NSF).
Acknowledging 2MASS in publications:
Please include the following in any published material that makes use
of the 2MASS data products:
"This publication makes use of data products from the Two Micron All
Sky Survey, which is a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Center/California Institute of
Technology, funded by the National Aeronautics and Space Administration
and the National Science Foundation."
File Summary:
File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file
out.sam 382 1000 2MASS Point Source Catalogue, sample output
(on a total of 470,992,970 sources)
See also:
http://www.ipac.caltech.edu/2mass/ : 2MASS documentation
VII/233 : The Two Micron All-Sky Survey: Extended sources (Cutri+ 2003)
Nomenclature Notes:
As specified by the IAU recommendations on source nomenclature, the
object designation is derived from the J2000 sexagesimal coordinates
of the source and has the form HHMMSSss±DDMMSSs, where HH are the
hours of right ascension, MM the RA minutes and SSss are the
centi-seconds of RA. DD are the degrees in declination, MM are DEC
minutes and SSs the declination deci-seconds. Note that both the RA
centi-seconds of time and DEC deci-seconds of arc are truncated rather
than rounded, per the IAU recommendation.
Note that when several sources have the same identifier, they are
distinguished by a trailing letter: A, B, C, etc...
Remark on the Byte-by-byte Description:
In the Byte-by-byte Description below, the column names differ
slightly compared to the original description, mainly to follow the
conventions used in the CDS Standaridzed description of catalogues.
The original names are written (in parentheses) in the Explanations.
Byte-by-byte Description of file: out.sam
Bytes Format Units Label Explanations
1- 10 F10.6 deg RAdeg (ra) Right ascension (J2000)
12- 21 F10.6 deg DEdeg (dec) Declination (J2000) (dec)
23- 26 F4.2 arcsec errMaj (err_maj) Semi-major axis of position
error ellipse
28- 31 F4.2 arcsec errMin (err_min) Semi-minor axis of position
error ellipse
33- 35 I3 deg errPA [0,180] (err_ang) Position angle of error
ellipse major axis (E of N)
37- 53 A17 --- 2MASS (designation) Source designation (1)
--------------------------------------------------------------------------------
55- 60 F6.3 mag Jmag ?(j_m) J selected default magnitude (2)
62- 66 F5.3 mag Jcmsig ?(j_cmsig) J default magnitude uncertainty (3)
68- 72 F5.3 mag e_Jmag ?(j_msigcom) J total magnitude uncertainty (4)
74- 83 F10.1 --- Jsnr ?(j_snr) J Signal-to-noise ratio
85- 90 F6.3 mag Hmag ?(h_m) H selected default magnitude (2)
92- 96 F5.3 mag Hcmsig ?(h_cmsig) H default magnitude uncertainty (3)
98-102 F5.3 mag e_Hmag ?(h_msigcom) H total magnitude uncertainty (4)
104-113 F10.1 --- Hsnr ?(h_snr) H Signal-to-noise ratio
115-120 F6.3 mag Kmag ?(k_m) K selected default magnitude (2)
122-126 F5.3 mag Kcmsig ?(k_cmsig) K default magnitude uncertainty (3)
128-132 F5.3 mag e_Kmag ?(k_msigcom) K total magnitude uncertainty (4)
134-143 F10.1 --- Ksnr ?(k_snr) K Signal-to-noise ratio
--------------------------------------------------------------------------------
145-147 A3 --- Qflg (ph_qual) JHK Photometric quality flag (5)
149-151 A3 --- Rflg (rd_flg) Source of JHK default mag (6)
153-155 A3 --- Bflg (bl_flg) JHK components fit to source (7)
157-159 A3 --- Cflg (cc_flg) Artifact contamination, confusion (8)
161-166 A6 --- Ndet [0-9] (ndet) Number of aperture measurements
(jjhhkk) (9)
--------------------------------------------------------------------------------
168-172 F5.1 arcsec prox Distance between source and nearest neighbour
174-176 I3 deg pxPA ? (pxpa) Position angle of vector from source
to nearest neighbour (E of N)
178-187 I10 --- pxCntr (pxcntr) Sequence number of nearest neighbour
189 I1 --- Xflg [0,2] (gal_contam) Extended source
contamination (10)
191 I1 --- Aflg [0,1] (mp_flg) Association with asteroid
or comet (11)
--------------------------------------------------------------------------------
193-202 I10 --- Cntr (pts_key) Unique source identifier in catalogue
204 A1 --- Hemis [ns] (hemis) Hemisphere of observation
206-215 A10 --- Date (date) Observation date
217-219 I3 --- Scan (scan) Scan number (within date)
221-227 F7.3 deg GLON (glon) Galactic longitude (12)
229-235 F7.3 deg GLAT (glat) Galactic latitude (12)
237-242 F6.1 arcsec Xscan (x_scan) Distance of source from focal plane
centerline
244-255 F12.4 d JD (jdate) Julian date of source measurement (13)
--------------------------------------------------------------------------------
257-262 F6.2 --- Jpsfchi ?(j_psfchi) J band reduced chi2 value of fit
264-269 F6.2 --- Hpsfchi ?(h_psfchi) H band reduced chi2 value of fit
271-276 F6.2 --- Kpsfchi ?(k_psfchi) K band reduced chi2 value of fit
278-283 F6.3 mag Jstdap ?(jmstdap) J standard aperture magnitude (18)
285-289 F5.3 mag e_Jstdap ?(jmsigstdap) error on Jstdap
291-296 F6.3 mag Hstdap ?(hmstdap) H standard aperture magnitude (18)
298-302 F5.3 mag e_Hstdap ?(hmsigstdap) error on Hstdap
304-309 F6.3 mag Kstdap ?(kmstdap) K standard aperture magnitude (18)
311-315 F5.3 mag e_Kstdap ?(kmsigstdap) error on Kstdap
--------------------------------------------------------------------------------
317-321 I5 arcsec edgeNS (distedgens) Distance from the source to the
nearest North or South scan edge
323-325 I3 arcsec edgeEW (distedgeew) Distance from the source to the
nearest East or West scan edge
327-328 A2 --- edge [nsew] (distedgeflg) flag indicating to which
edges the edgeNS and edgeEW values refer
330 I1 --- dup (dup_src) Flag indicating duplicate source (14)
332 I1 --- use (use_src) Use source flag (15)
--------------------------------------------------------------------------------
334 A1 --- opt [0UT] (a) Associated optical source (16)
336-338 F3.1 arcsec Dopt ?(dist_opt) Distance to associated optical
source
340-342 I3 deg PAopt [0,360]? (phi_opt) position angle from optical
source to the 2MASS source position
344-348 F5.2 mag Bmag ?(bmopt) Blue magnitude of associated
optical source
350-354 F5.2 mag Rmag ?(vrmopt) Visual or red mag of associated
optical source
356 I1 --- Nopt (nopt_mchs) Number of optical sources
within 5arcsec (17)
--------------------------------------------------------------------------------
358-364 I7 --- extKey ?(ext_key) Record Identifier in XSC
366-370 I5 --- scanKey (scan_key) Record Identifier in the
Scan Information Table
372-378 I7 --- coaddKey (coadd_key) Record Identifier in the
Atlas Image Data Table
380-382 I3 --- coadd (coadd) Sequence number of the Atlas Image
Note (1): Sexagesimal, equatorial position-based source name in the form:
hhmmssss+ddmmsss[ABC...].
The full naming convention for 2MASS All-Sky Release PSC sources has
the form "2MASS Jhhmmssss+ddmmsss[ABC...]", where "2MASS" indicates
that the source is from the All-Sky PSC, "J" indicates the position is
J2000, and the optional trailing letters are used to distinguish
sources with otherwise duplicate names. The "2MASS J" prefix is not
listed explicitly in the designation column.
Note (2): This is the selected "default" magnitude for each band, [JHK].
If the source is not detected in the band, this is the 95% confidence
upper limit derived from a 4" radius aperture measurement taken at the
position of the source on the Atlas Image. The origin of the default
magnitude is given by the first character of the Rflg value
(Rflg). This column is null if the source is nominally detected in
the band, but no useful brightness estimate could be made
(Rflg="9").
Note (3): This is the corrected photometric uncertainty [JHK]cmsig for
the default magnitude. If Rflg="2", this is the measurement error
from the profile-fitting procedure, corrected to be consistent with
observed repeatability statistics. If Rflg="1" or "4", this is the
RMS of the brightness measured in aperture photometry on the
individual frames. If Rflg="3", the uncertainty is derived from the
residuals to the 1-d radial profile fit to the unsaturated wings of
the bright star.
If the value is >8.0, it is a flag value indicating that a meaningful
uncertainty could not be determined for the source. This column is
null if the default magnitude is a 95% confidence upper limit
(Rflg="0" or "6") or if the source is not measurable (Rflg="9").
Note (4): Combined, or total photometric uncertainty [JHK]msigcom
for the default magnitude in that band. The combined uncertainty is
derived from the following relation:
e_[JHK]mag = sqrt([JHK]cmsig2 + [JHK]zperr2
+ fferr2 + [r1normrms2])
where
cmsig = Corrected band photometric uncertainty
zperr = Nightly photometric zero point uncertainty = 0.011 mag
fferr = Flat-fielding residual error = 0.005 mags
r1normrms = R1 normalization uncertainty = 0.012 mags
(applied only for sources with Rflg="1")
This column is null if the default magnitude is a 95% confidence upper
limit (i.e. the source is not detected, or inconsistently deblended in
the band).
Note (5): Three character flag, one character per band [JHK],
that provides a summary of the net quality of the default photometry
in each band, as derived from the Read Flag (Rflg), measurement
uncertainties ([jhk]cmsig), scan signal-to-noise ratios ([jhk]snr),
frame-detection statistics (Ndet), and profile-fit reduced chi-squared
values ([jhk]psfchi). The value for Qflg is set for a band
according to the precedence of the table below. For example, a source
that is tested and meets the conditions for category "X" is not tested
for subsequent qualities.
X = There is a detection at this location, but no valid brightness
estimate can be extracted using any algorithm. Rflg="9" and
default magnitude is null.
U = Upper limit on magnitude. Source is not detected in this band
(Rflg="0"), or it is detected, but not resolved in a consistent
fashion with other bands (Rflg="6"). A value of Qflg="U"
does not necessarily mean that there is no flux detected in this
band at the location. Whether or not flux has been detected can
be determined from the value of Rflg. When Rflg="0", no flux
has been detected. When Rflg="6", flux has been detected at the
location where the images were not deblended consistently in all
three bands (JHKs).
F = This category includes Rflg="1" or Rflg="3" sources where a
reliable estimate of the photometric error, [jhk]cmsig, could
not be determined. The uncertainties reported for these sources
in [jhk]cmsig and e_[jhk]mag are flags and have numeric
values >8.0.
E = This category includes detections where the goodness-of-fit
quality of the profile-fit photometry was very poor (Rflg=2 and
[jhk]psfchi>10.0), or detections where psf fit photometry did
not converge and an aperture magnitude is reported (Rflg=4), or
detections where the number of frames was too small in relation
to the number of frames in which a detection was geometrically
possible (Rflg="1" or Rflg="2").
A = Detections in any brightness regime where valid measurements
were made (Rflg="1","2" or "3") with
[jhk]snr>10 AND [jhk]cmsig<0.10857.
B = Detections in any brightness regime where valid measurements
were made (Rflg="1","2" or "3") with
[jhk]snr>7 AND [jhk]cmsig<0.15510.
C = Detections in any brightness regime where valid measurements
were made (Rflg="1","2" or "3") with
[jhk]snr>5 AND [jhk]cmsig<0.21714.
D = Detections in any brightness regime where valid measurements
were made (Rflg="1","2" or "3") with
no [jhk]snr OR [jhk]cmsig requirement.
Note (6): The read flag indicates the source of J,H,K "default" magnitudes
as a 3-digit integer, where the 1st digit corresponds to the J band,
the 2nd to the H band, and the 3rd to the Ks band. Rflg values of
"1", "2" or "3" generally indicate the best quality detections,
photometry and astrometry (although other quality flags must be
considered). Values of "0", "4", "6" and "9" in a band indicate
either non-detections, or generally poor quality photometry and
positions. The definitions of the Rflg values are:
0 = Source is not detected in this band. The default magnitude
is the 95% confidence upper limit derived from a 4" radius
aperture measurement taken at the position of the source on the
Atlas Image. The sky background is estimated in an annular region
with inner radius of 14" and outer radius of 20".
1 = The default magnitude is derived from aperture photometry
measurements on the 51 ms "Read_1" exposures. The aperture radius
is 4", with the sky background measured in an annulus with an
inner radius of 14" and an outer radius of 20". Used for sources
that saturate one or more of the 1.3s "Read_2" exposures, but are
not saturated on at least one of the 51 ms "Read_1" frames.
2 = The default magnitude is derived from a profile-fitting
measurement made on the 1.3 sec "Read_2" exposures. The
profile-fit magnitudes are normalized to curve-of-growth
corrected aperture magnitudes. This is the most common type in
the PSC, and is used for sources that have no saturated pixels in
any of the 1.3 sec exposures.
3 = The default magnitude is derived from a 1-d radial profile
fitting measurement made on the 51 ms "Read_1" exposures. Used
for very bright sources that saturate all of the 51 ms "Read 1"
exposures.
4 = The default magnitude is derived from curve-of-growth-corrected
4" radius aperture photometry measurements on the 1.3 s "Read_2"
exposures. This is used for sources that are not saturated in any
of the Read_2 frames, but where the profile-fitting measurements
fail to converge to a solution. These magnitudes are the same as
the standard aperture magnitudes (Jstdap, Hstdap, Kstdap), but
when they are the default magnitudes, it generally implies that
they are low quality measurements.
6 = The default magnitude is the 95% confidence upper limit derived
from a 4" radius aperture measurement taken at the position of the
source on the Atlas Image. The sky background is estimated in an
annular region with inner radius of 14" and outer radius of 20".
This is used for pairs of sources which are detected and resolved
in another band, but are detected and not resolved in this band.
This differs from a Rflg="0" because in this case there is a
detection of the source in this band, but it is not consistently
resolved across all bands.
9 = The default magnitude is the 95% confidence upper limit derived
from a 4" radius aperture measurement taken at the position of the
source on the Atlas Image. The sky background is estimated in an
annular region with inner radius of 14" and outer radius of 20".
This is used for sources that were nominally detected in this
band, but which could not have a useful brightness measurement
from either profile fitting or aperture photometry. This often
occurs in highly confused regions, or very near Tile edges where
a significant fraction of the measurement aperture of sky annulus
falls off the focal plane.
Note (7): Blend flag. Three character flag, one character per band [JHK],
that indicates the number of components that were fit simultaneously
when estimating the brightness of a source:
0 = Source is not detected, or is inconsistently deblended in that
band.
1 = One component was fit to the source in R_2 profile-fitting
photometry (Rflg="2"), or default magnitudes are from aperture
photometry (Rflg="1" or "4") or saturated star 1-d radial
profile-fitting (Rflg="3").
>1 = More than one component was fit simultaneously during R2
profile-fit photometry, where the value of the field is the
number of components simultaneously fit. The maximum number of
components is 7 in any band for the PSC, so this Bflg is always
a three character flag. Multi-component fitting occurs only for
profile-fitting, and only when more than one detection is found
within ∼5". Single detections that are not well-fit by a single
PSF are not split.
Note (8): Contamination and confusion flag. Three character flag, one
character per band [JHKs], that indicates that the photometry and/or
position measurements of a source may be contaminated or biased due to
proximity to an image artifact or nearby source of equal or greater
brightness. The Cflg in each band is set in hierarchical order
according to the following table, in the event a source is affected by
more than one artifact or condition:
p = Persistence. Source may be contaminated by a latent image left
by a nearby bright star.
c = Photometric Confusion. Source photometry is biased by a nearby
star that has contaminated the background estimation. This is
very common in high source density regions.
d = Diffraction spike confusion. Source may be contaminated by a
diffraction spike from a nearby star.
s = Electronic stripe. Source measurement may be contaminated by a
stripe from a nearby bright star.
b = Bandmerge confusion. In the process of merging detections in
the different bands for this source, there was more than one
possible match between the different band components. This occurs
in regions of very high source density, or when multiple sources
were split in one band but not another.
0 = Source is unaffected by known artifacts, or is not detected in
the band.
Non-zero values the Cflg in any band is an indicator that the
measurements of that source may be contaminated.
Note (9): ndet is a six-character flag, two characters per band [JJHHKsKs],
that indicates the number frames on which a source was detected, N,
and the number of frames on which the source could have been measured,
M. The first digit in the pair for each band, N, gives the number of
frames on which there were >3 sigma aperture photometry detections of
the source. The second digit for each band, M, gives the number of
frames on which aperture measurements were possible. Normally, M=6,
(occasionally M=7 because the scanning step size was slightly less
than 1/6 of the frame size). M can be <6 if frames are "lost" because:
a) there were masked pixels within the source aperture due to cosmic
ray detections, noisy pixels, meteor trails, etc, or b) there were
saturated pixels within the source aperture. Values of M<6 are not
uncommon, but do indicate an effective loss of coverage and
sensitivity for a source.
The values of N and M for a band always pertain to the aperture
photometry on the 51 ms "Read_1" exposures if Rflg="1", and to the
aperture photometry on the 1.3 s "Read_2" exposures if Rflg="2" or
"4". Sources that are saturated on all of the Read_1 frames
(Rflg="3") have N=0 because no aperture photometry was possible.
Sources with signal-to-noise ratios >8-9 should be detected on all
available frames. Values of N < M for such sources are an indication
of possible loss of measurement integrity. Values of N < M, or even
N=0, are normal for fainter sources because they are too faint on
individual frames to yield a >3 sigma detection.
Note (10): Extended source "contamination" flag. A value of
Xflg="2" indicates that this point source falls within the elliptical
boundary of an XSC source defined by the semi-major axis and
ellipticity of the 20 mag.arcsec-2 Ks-band elliptical isophote
(r_k20fe)+10%, for XSC sources where r_k20fe is >10''. Sources so
indicated are often foreground stars superimposed on background
galaxies, or sometimes extractions of pieces of the galaxy or nebula.
The point source photometry for these sources is probably contaminated
by the surrounding structured extended emission. Point sources can
still be superimposed on extended sources and remain unflagged if the
XSC source shape is not well-described by an ellipse.
This flag also denotes (Xflg="1") PSC sources that are equivalent to
sources in the XSC, but only for XSC sources with r_k20fe is >10''.
Since most XSC sources have semi-major axes <10'', Xflg does not
identify all PSC sources with exact XSC counterparts. Use the extKey
to identify all PSC sources that were found to be resolved relative to
a single point-spread function.
0 = Source does not fall within the elliptical profile of an extended
source with semi-major axis >10'', or it is not identified
exactly with an XSC source with semi-major axis >10''. However,
the source may correspond exactly to a smaller XSC source.
1 = Source is resolved by 2MASS, and is equivalent to a source in the
XSC that has a semi-major axis >10'' in size. Users must refer to
the extKey to identify all PSC sources with XSC counterparts.
2 = Source falls within the elliptical boundary of an XSC source that
has a semi-major axis >10'' in size.
Note (11): Minor Planet Flag. Indicates if this source is associated
with the predicted position of a known minor planet, comet, planet or
planetary satellite. This association does not guarantee that the PSC
source is a detection of the solar system object.
0 = Source is not associated with a known solar system object
1 = Source is associated with the predicted position of a known
solar system object. The name and orbital data for the associated
objects are given in the Known Asteroid Detection List, Known
Comet Detection List, and Planet and Planetary Satellite
Detection List. The associated PSC source is cross-referenced in
those Lists via the value of ptsKey.
Note (12): Galactic coordinate for 2MASS source derived by precessing
reconstructed ra and dec to B1950, and performing rotational
transformation into lII, bII coordinate system. This coordinate should
not be used as an astrometric reference because it has been rounded to
0.001 deg.
Note (13): The Julian Date of the source measurement is accurate to
±30seconds. This value is extrapolated from the start time of the
Survey scan using the difference between the declinations of the
source and the first row in the Tile divided by the scanning rate of
the telescope (approximately 5sec). The scanning rate of the two 2MASS
telescopes was slightly different because of the need to optimize the
dithering of images on the arrays.
Note (14): Used in conjunction with the use flag, this numerical flag
indicates whether the source falls in a Tile overlap region, and if
so, if it was detected multiple times.
0 = The source does not fall within a Tile overlap region
(use="1"), or it falls in an overlap region and there were no
sources detected within 2" of its position in the overlapping
Tile (use="0").
1 = The source falls within a Tile overlap region, and there is one
source detected within 2" of its position in the overlapping Tile.
>1 = The source falls within a Tile overlap region, and there are
multiple sources detected within 2" of its position in the
overlapping Tile, and/or within its own Tile. This implies
confusion in the multiple source resolution procedure.
Note (15): Used in conjunction with the dup flag, this numerical flag
indicates if a source falls within a Tile overlap region, and whether
or not it satisfies the unbiased selection rules for multiple source
resolution.
1 = The source does not fall in a Tile overlap region
(dup="0"), or it falls in an overlap region, but lies farther
from its scan edge than any apparition of the source in another
Tile. A source must have use="1" to be considered part of the
sub-Catalog that satisfies the 2MASS Level 1 Science Requirements
0 = The source falls in a Tile overlap region, but does not have
a detection within 2" of its position in any overlapping Tiles.
PSC sources with use="0" will always have dup="0". Such
objects are usually faint, and thus toggling above and below the
detection limits, or may have moved between the scans of the
different Tiles.
Note (16): The catalog ID and other association parameters are given for
the closest optical match found within 5 arcsec. The ID consists of a
single-letter abbreviation for the catalog (e.g., 'T' for Tycho 2, 'U'
for USNO-A2.0). '0' means no optical source was found.
Note (17): The optical association information is unreliable for sources
with |dec|>86°, because of an error in the 2MASS/USNO-A2.0 position
correlation procedure that caused associations to be missed in the
vicinity of the equatorial poles. If the source has |dec|>86°, a
value of a="0" does not necessarily mean that there is not an optical
association within 5 arcsec of the 2MASS position.
Note (18): the standard aperture magnitude is the curve-of-growth corrected
average brightness measured in a 4" radius aperture centered on the
source position on each of the available 1.3 s "Read_2" frames. The
column is empty if the source is not detected in the band (Rflg="0"),
or if it is saturated in all of the 1.3 s "Read 2" exposures for the band.
History:
* 10-Jun-2003: this ReadMe document was prepared from the documents at
http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec2_2a.html(End) Sebastien Derriere, Francois Ochsenbein [CDS] 18-Jun-2003